GB2041975A - Te and plutonium and/or uranium ions solidifying a solution containing ammonium nitrate carbona - Google Patents

Description

1 GB 2 041 975A 1

SPECIFICATION

Method and apparatus for processing a solution containing ammonium, nitrate, carbonate, and uranium and/or plutonium ions This invention relates to a method of processing a solution containing ammonium, nitrate and carbonate ions and either or both of uranium and plutonium ions.

This invention more particularly but not exclusively relates to a method of reprocessing radioactive filtrates containing ammonium ni- trate, as are obtained, for example, in the Ammonium Uranyl Carbonate (AUC) process or the Ammonium Uranyl Plutonium Carbonate (AUPuC) process. In the AUC process, which is described in greater detail in German Offen legungssch rifts 1 952477 and 15 9 24 7 1, there are obtained filtrates which have the following composition if uranyl nitrate was the starting product:

c. 100 9/1 NH4 c. 150 g/1 N03 C. 90 9/1 C03 C. 300 mg/i U The filtrates from the AUPuC process are similar, except that in addition to the uranium content there is also a portion of plutonium.

When non-radiated and plutonium-free uranyl nitrate is used as starting product, after sufficient chemical separation of uranium and its decay products, the filtrate from the production of nuclear fuel can be released into an ordinary sewer system without generating too high a radiological burden on the environ- ment.

However, this is not possible if irradiated uranium or plutonium is contained in the filtrate. Decontamination using chemical means is no longer sufficient. Such a filtrate is produced after processing irradiated fuel elements.

The volume of filtrate originating from the production of nuclear fuel is comparatively large, with the result that it cannot practically be released for final storage until its volume has been reduced. It is necessary to try for the greatest possible reduction in volume, for example by solidification.

Total evaporation cannot be permitted due to the danger of explosion as a result of the ammonium nitrate in this connection. However, in order to reduce the volume, the water must be separated from the radioactive constituents, thus the ammonium nitrate must be decomposed.

There are several known methods for the thermal decomposition of the ammonium nitrate at temperatures above 250', the water being evaporated with the decomposition products. The radioactive constituents remain in the reaction vessel during these methods.

However, these methods have the disadvantage of being able to decompose the filtrates only if these were evaporated before-hand until the NH, N03 content is approximately 75-80%. However, the risk of explosion is not excluded in this technique. Furthermore, they produce decomposition products which either cannot be recycled or can be re-used only at great expense. Furthermore, the relatively high temperature level makes these thermal processes quite expensive; it should also be mentioned that the necessary cleansing of the apparatus of the radioactive constit- uents (plutonium dust) also poses problems.

According to one aspect of the present invention there is provided a method of processing a solution containing ammonium, nitrate and carbonate ions, and uranium and/or plutonium ions and method comprising supplying the solution to an electrolytic cell containing an ammonium nitrate solution and passing an electric current through the liquid in the cell so that the ammonium ions and carbonate ions produce gaseous ammonia and carbon dioxide respectively, the nitrate ions are reduced to gaseous ammonia, and the uranium and/or plutonium ions are precipitated, the gaseous ammonia and carbon diox- ide together with water vapour from the solution being drawn off from the electrolytic cell. The uranium may be precipitated as diuranate (hydroxide).

The uranium and/or plutonium may be deposited electrolytically at the cathode.

The electric current flowing in the electrolytic cell can provide heat necessary to maintain the solution at its boiling point.

Preferably, the electrolytic cell initially con- tains ammonium nitrate and the first-mentioned solution is continuously supplied to the electrolytic cell.

Preferably the ammonium nitrate solution and the first-mentioned solution contain cop- per.

Preferably the rate at which the first-mentioned solution is supplied to the cell and the electric current in the cell are adjusted so that the rate at which ammonia, carbon dioxide and water are evolved in the cell is approximately equal to the rate of supply of the firstmentioned solution.

According to another aspect of the present invention there is provided an apparatus, for carrying out the method according to the present invention, the apparatus comprising a vessel formed as a body of rotation, a cylindrical cathode, and an annular anode, both the cathode and the anode being coaxial with the vessel.

Alternatively the apparatus according to the present invention can comprise: an electrolytic cell which includes a vessel and within the vessel an anode and a cathode, which are arranged so that, in use, the cathode current 2 GB2041975A 2 density is greater than the anode current density; a recirculation line connecting an outlet in a lower region of the vessel to an upper region of the vessel, in which recircula- tion line there are a pumping means and a filter for removing precipitated uranium and/ or plutonium; and an inlet line for solution to to be processed which inlet line opens into the vessel at a location adjacent the cathode.

The apparatus can include a dividing wall which comprises a cylindrical portion and a circular portion, which separates a cathode zone adjacent the cathode from an anode zone adjacent the anode, the portion of the dividing wall above the intended liquid level being impervious to gas, and at least part of the portion of the dividing wall below the intended liquid level being pervious to the solution.

The apparatus can further include a cover, which covers the top of the vessel and to which the dividing wall is attached so that, in use, with the vessel filled with solution, a cylindrical region for gas is defined above the cathode, and an annular region for gas is defined above the anode.

For a better understanding of the present invention and to show more clearly how the same may be carried into effect reference will now be made, by way of example, to the accompanying drawing which shows diagrammatically an apparatus according to the present invention.

An electrolytic cell 1 comprises a cylindrical vessel which is provided with an annular anode 2, for example of graphite granules, coated titanium or iron, and a rod-shaped, profiled cathode 3 of high-quality steel. A cathode space 31 around the cathode 3 is separated from an anode space 21 adjacent the anode 2 by a cylindrical, dividing wall 11. The upper portion of the dividing wall 11 from a cover of the electrolytic cell 1 to a few centimeters below the surface 8 of a liquid is impervious and comprises a high-quality steel, and a lower portion of the dividing wall 11, which is connected to the upper portion comprises a chemically stable, porous, insulating material, for example polypropylene fabric, and extends downwards to beyond the lower limit of the electrodes 2 and 3.

There is located in the anode space 21 a heat exchanger 6 which is connected on the input side to a supply line 7 for the solution to be processed, and on the output side via a line 76 to an input 32 of the cathode space 31. There is also connected to the input 32 a recirculation line 53 which leads from a filter 5 which, in turn, is connected via a pump 4 to an outlet 24. The anode space 21 which, like the cathode space 31, is filled only to slightly above the anode 2 with an electrolyte 8 is provided with gas outlet lines 22, and the cathode space 31 is provided with a gas outlet line 33. The line 33 and the lines 22 open into a cathode region above the cathode 3 and an anode region above the anode 2 respectively. Finally, there is an outlet valve 12 in the base of the electrolytic cell 1, and lines 23 for an air supply connected to the outlet lines 22.

The method according to the present invention will now be described.

A boiling ammonium nitrate solution con- taining, uranium and/or plutonium carbonate complexes with a concentration of approximately 250 g/1 is located as an electrolyte 8 in the cathode and anode spaces 21 and 31 of the cell 1. A filtrate intended for reprocess- ing is then supplied through the line 7. Before entering the electrolytic cell 1, the filtrate first passes through the heat exchanger 6, is preheated there and then flows through the line 76 and input 32 to the cathode space 31.

The pre-heating can be still further improved by a heat-exchanging connection (not shown) with the gases drawn off through the line 33.

Initially, some copper is added to this filtrate so that there is no disturbing cathodic hydrogen generation at the cathode 3, the cathode 3 then virtually acts as a copper electrode. By adding copper to the filtrate, the copper character of the cathode 3 is constantly maintained since repeated separation of copper occurs as a result of partial detachment of the copper coating from the cathode 3.

The filtrate supplied through the input connection 32 now also begins to boil, thereby releasing its ammonium carbonate and free NH, content in the form of gaseous CO, and NH,. These gases are drawn off together with the water vapour produced through boiling via the line 33 and advantageously are supplied to a nuclear fuel production process. The filtrate thereby becomes a sightly uranium- or plutonium-containing ammonium nitrate solution. Since the solubility of the NH3 at approximately 1 00C is small, a pH value of approxi- mately 6-7 is automatically established. With this pH, the likely COT concentration is extremely low. As a result, the uranium and/or plutonium which was/were previously in solution as carbonate compound(s) is/are depos- ited as diuranate (NH4)2U207' The entire contents of the electrolytic cell 1 are circulated by the pump 4, the deposit continuously obtained being removed with ease by the filter 5 disposed in the pump line.

In this connection the pumping direction, as marked, is from the anode space 21 into the cathode space 31. Since the expulsion of the C02 is not 100 %, a small quantity of uranium (plutonium) remains dissolved. However, this dissolved uranium is cathodically sepeated by electrolysis at the cathode 3 and can be detached from the cathode 3 with the aid of acids during pauses in the operation of the cell 1.

Apart from the slight uranium sepeation, 3 GB 2 041 975A 3 the electrolytic process causes the cathodic reduction of the N03 to NH3 which escapes from the boiling solution. The water is decomposed to 02 at the anode 2. The otherwise undesirable heat obtained with electrolysis is deliberately utilized here in order to maintain the bath in the boiling state, to decompose the ammonium carbonate, to expel the ammonia and to evaporate the solvent water.

The heat generated by electrolysis is regulated by the cell voltage-this amounts to a few volts-such that the volume of liquid being evaporated corresponds to the quantity of ammonium nitrate decomposed which is present. The volume of liquid evaporated likewise corresponds to the quantity of filtrate supplied. The method thus operates fully, continuously, with constant concentrations and conversion rates.

Since, as already mentioned, anodic oxygen is generated and, with NH3, this could produce explosive mixtures, the electrolytic cell 1 is constructed in the manner shown. This construction with the anode 2 around the cathode 3 causes the anodic current density in the anode space 21 to be smaller than the current density in the cathode space 31, with the result that only the cathode space 31 is in the boiling state. Since the fresh filtrate is supplied only to the cathode space 31, and the reduction of the N03 to NH3 also takes place theree, NH3 only escapes here. The NI-1, C02 and water vapour are drawn off through the line 33. Since the cathode space is separated from the anode space by the dividing wall 11, the oxygen generated at the anode 2 is safely prevented from mixing with NH,. The oxygen is drawn off via the lines 22 and diluted with additional air from the lines 23. The wall 11 prevents an explosive mixture of NH, and 02 being formed.

The safety aspect already referred to, namely avoiding the risk of explosion is also taken into account in this method and the apparatus shown, in that the electrodes 2 and 3 extend only approximately halfway down the electrolytic cell 1. This ensures that in the event of a possible interruption in the filtrate supply via the line 7 and also other disruption or malfunction of the other regulating elements (not shown), the concentration of the solution located in the vessel can only be doubled by evaporation, since, after half the solution is evaporated, the electrodes are then immersed no further into the solution, and evaporation then automatically ceases. However, this concentration is not completely without danger.

Finally, it should be reiterated that the tem- perature level is only approximately 1 OWC, in contrast to temperature of 25WC in the thermal method which was initially discussed. The heat transfer is direct and therefore virtually without loss. The radioactive constituents are separated in a simple and dust-free manner in the filter 5 and can be withdrawn there in a known manner. The resultant gaseous reaction products can be released into the atmosphere without danger or can be recycled, i.e.

fed into a nuclear fuel production process for example an AUC process. The deposition products taken from the filter 5 already have, compared to the starting solution, an extremely great reduction in volume and can be stored as a radioactive waste possibly after further solidification or recycled. The electrolytically separated uranium or plutonium respectively on the cathode 3 is supplied to the nuclear fuel production process again in a known manner.

Claims (25)

1. A method of processing a solution containing ammonium, nitrate and carbonate ions, and uranium and/or plutonium ions, the method comprising supplying the solution to an electrolytic cell containing an ammonium nitrate solution and passing an electric current through the liquid in the cell so that the ammonium ions and carbonate ions produce gaseous ammonia and carbon dioxide respectively, the nitrate ions are reduced to gaseous ammonia, and the uranium and/or plutonium ions are precipitated, the gaseous ammonia and carbon dioxide together with water vapour from the solution being drawn off from the electrolytic cell.

2. A method as claimed in claim 1, wherein uranium and/or plutonium is/are also deposited electrolytically at the cathode.

3. A method as claimed in claim 1 or 2, wherein the electric current flowing in the electrolytic cell provides heat necessary to maintain the solution at its boiling point.

4. A method as claimed in claim 3, wherein the electrolytic cell initially contains an ammonium nitrate solution ' and wherein the first- mentioned solution is continuously supplied to the electrolytic cell.

5. A method as claimed in claim 4, wherein the ammonium nitrate solution an d/or the first-mentioned solution contain cop per.

6. A method as claimed in claim 4 or 5, wherein the rate at which the first-mentioned solution is supplied to the cell and the electric -current in the cell are adjusted so that the rate at which ammonia carbon dioxide and water are evolved in the cell is approximately equal to the rate of supply of the first mentioned solution.

7. A method as claimed in claim 4, 5 or 6, wherein the first-mentioned solution is preheated before it is supplied to the electrolytic cell.

8. A method as claimed in claim 7, wherein the first-mentioned solution is preheated in a heat exchanger which is disposed within the electrolytic cell.

9. A method as claimed in any one of 4 GB2041975A 4 claims 4 to 8 wherein the first-mentioned solution is supplied to a cathode zone adjacent a cathode of the electrolytic cell.

10. A method as claimed in any preceed- ing claim, wherein the solution in the electrolytic cell is circulated by a pumping means which draws off the solution from the cell and pumps the solution through a filter back into the cell, any uranium and plutonium deposits being filtered off in the filter.

11. A method as claimed in claim 10, wherein appendant to claim 9, wherein the pumping means. pumps the solution into the cathode zone in the cell.

12. A method as claimed in any preceding claim, wherein the solution is a radioactive filtrate from a nuclear fuel production process.

13. A method as claimed in claim 12, wherein the nuclear fuel production process is an ammonium uranyl carbonate process or an ammonium uranyl plutonium carbonate process.

14. A method as claimed in any preceding claim, wherein any oxygen generated at an anode of the electrolytic cell is separated from the carbon dioxide and ammonia that are evolved in the cell.

15. A method as claimed in claim 14, wherein the ammonia and carbon dioxide are retained for further use.

16. A method substantially as hereinbefore described with reference to the accompanying drawing.

17. An apparatus, for carrying out the method as claimed in claim 1, the apparatus comprising a vessel formed as a body of rotation, a cylindrical cathode, and an annular anode, both the cathode and the anode being coaxial with the vessel.

18. An apparatus comprising: an electrci lytic cell which includes a vessel and within the vessel an anode and a cathode which are arranged so that, in use, the cathode current density is greater than the anode current density; a recirculation line connecting an outlet in a lower region of the vessel to an upper region of the vessel, in which recirculation line there are pumping means and a filter for removing precipitated uranium and/or plutonium; and an inlet line for solution to be process which inlet line opens into the vessel at a location adjacent the cathode-

19. An apparatus as claimed in claim 17 or 18, which also includes a dividing wall which comprises a cylindrical portion. and a circular portion, which separates a cathode zone adjacent the cathode from an anode zone adjacent the anode, the portion of the dividing wall above the intended liquid level being impervious to gas, and at least part of the dividing wait below the intended liquid level being pervious to the solution.

20. An apparatus as claimed' in claim, 19, which further includes a cover, which covers the top of the vessel and to which. the divid- ing wall is attached so that, in use, with the vessel filled with solution, a cylindrical region for gas is defined above the cathode, and an annular region for gas is defined above the anode.

21. An apparatus as claimed in claim. 20, wherein the cover is provided with an inlet for the first-mentioned solution, which inlet opens into the cylindrical region above the cathode.

22. An apparatus as claimed in claim 21, which includes an inlet line for the firstmentioned solution which is connected via a heat exchanger, disposed in the anode zone of the vessel to the inlet.

80,

23. Art apparatus as claimed in claim 21 or 22 when appendant to claim 17 which includes an outlet in a lower region of the vessel which is connected via a pump and a filter to the inlet and which opens into the anode zone.

24. An. apparatus as claimed in any one of claims 2G to 23, wherein the cylindrical region above the cathode is provided with an outlet in the cover through which in use ammonia, carbon dioxide and water vapour are drawn off from the cell, and the annular region above the anode is provided with one or more outlets in the cover for oxygen generated at the anode.

25. An apparatus substantially as hereinbefore described with reference to, and as shown in, the accompanying drawing.

Printed for Her Majesty's Stationery Office by Burgess & Son (Abingdon) Ltd.-1 980. Published at The Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained.

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